yamaha c115 tlr running issues

hi everyone. I am new to all this. I have a 99 Yamaha c115 TLR 2 stroke that keeps stalling and just not running right. Things I have done already: cleaned carbs, replaced fuel lines from tank to motor, replaced fuel pump, checked reeds, changed plugs, checked compression (140) on all 4 cylinders, added fuel/water separator, changed fuel filter, checked for air leaks, and attempted to adjust air mixture.

symptoms I am having: ran good for a while. Then it began to foul plugs. I would cruise at WOT speed around 40 mph. After running for about 20 minutes at WOT, i would lose top end like it was starving for fuel. I could back off to around 30 mph, and it would run fine. As I get back to WOT, it would run good for about 2 minutes, then lose the power again. The plugs were black with carbon foul. Asking around, i was told two different things about the air mix screw settings. One was two turns out for general start point, and the other was only 5/8 out from seated. I tried both. During adjustment, not much changed on the idle side of things. I ran the boat for a while in the water moving place to place and adjusting each time i stopped. Got it running good at WOT with RPM in range, but then starting having stalling at idle. Last thing i changed was fuel pump. Now it will run for a few minutes and then stall like its not getting fuel moving to the motor. Had a thought of putting an electric fuel pump to see if this changes anything. Seems to be an adjustment issue somewhere. what is the actual start point on the air mix screw? Or would this just be easier and better to pay several hundred dollars to put in the shop? Any help would be great. I'm just frustrated at this point and trying to save a little money where I can. First boat owned and its not going well. I'm starting to feel the analogy of B.O.A.T., break out another thousand....

Test run with another portable tank and hose.-----Have an assistant ready to operate the manual pump when motor falters.-----And does spark jump a gap of 3/8" on all leads , yes or no ?------Checked operation of your oil injection system ?----Checked / replaced water pump impeller ?----Checked operation of thermostats ?

Have not tried a portable tank. Will try that. Will also check spark. Does not have oil injection. It's premix 50:1. Water pump was changed a few months ago when i purchased the boat. Thermostats were removed prior to my purchase because I went to check them just for general maintenance. Would the thermostats pose an issue with starving for fuel?

I sometimes ask folks these questions to get an idea of what kind of attention / maintenance a motor has had.----I know that means absolutely nothing to you and may be considered a nuisance question ---Sorry , with a bit of luck you will have it running like new.

no worries. The questions don't bother me. At this point, anyone on here would know more than me. I just wasn't sure if they would be something i needed to put back in to run properly. I have had the boat only a few months. From the looks of the boat when i got it, it was not well taken care of. But I checked all the important stuff before purchasing. I know this is going to end up being a simple fix. I am just at a blind spot as to what else could be the issue.

YES removing the thermostats will cause condensation in the combustion chambers and put the spark out and cause power loss. The engine has to get to running temp.... Put the t-stats back in and run the engine, make sure both cylinder heads are getting up to temp.

Thanks. I ordered some last night to put back in. Should be here Sunday. I also learned yesterday that the store I was getting fuel at has water in the tanks. I am going to drain tank and clean everything out. I have a water separator, but some could have made it past that point. I have to work all weekend, but will update Monday or Tuesday once I drain tank, get fresh fuel, install thermostats, and change the plugs. Thanks for the help.

Also, I don't have a service manual for this motor. Can anyone tell me if the 2 full turns out on the air mix adjustment is correct for a starting point or what it is?

1.5 to 2 is fine.... This adjustment is only for idle. Not going to make the engine run different on the top end.

ok, so I put in thermostats, changed fuel pump, checked spark, and adjusted air screw to two turns out. idles fine, acceleration is kinda low. on top end, it is still starving for fuel. while running, if i squeeze the prime bulb, it picks up. almost like fuel pump is not pulling enough fuel. pick up tube has a screen on it, but it has nothing plugging it up. its clean. i have an electric fuel pump from my old boat. could this help or not a good idea? Also, while running, it will bog and serge as if it gets fuel to run, then loses it again. I'm waiting on an external fuel tank from a friend to try from a different tank. Until then, any other ideas as to what could be going on?

Did you replace the Mechanical fuel pump? If yes, did you install a new gasket? Do a compression test on all cylinders and pay attention to the cylinder that pulses the fuel pump.

Do not install an electric pump.

Yes. changed the mechanical fuel pump and gasket. I will do another compression test. Did one when I got the boat a few months ago. All cylinders were around 140. I'm assuming the cylinder closest to the fuel pump is the one that pulses the pump. Is that correct?

Yes, Check the fuel pump installation and make sure the gasket is in place... Easy to drop one side. Did you purchase a Yamaha pump or a cheap replacement?

FIXED...!!! Update, i am an idiot. When I changed the fuel pump, I tightened the bolts too tight and pushed out the inner gasket and split it. I did not notice with the wires and such in front of it. Didn’t notice until I took it back off to put old one back on. Rebuilt pump and all is well now. The fouling plug issue was adjustments and carb rebuild. Thanks for all help.